Aliphatic polyesters, particularly those obtained from hydroxycarboxylic acids, such as polylactic acids, polyglycolic acids and copolymers containing these polymers, attract attention as biodegradable polymer compounds. The biodegradable polymer compounds are used in various materials such as medical materials including sutures and sustained-release materials such as drugs, agricultural chemicals and fertilizers.
For use in such fields, the polymer compounds generally require high mechanical properties. Accordingly, the polymer compounds should have a high molecular weight. According to a conventional process for obtaining high-molecular weight polymers, lactide or glycolide is produced from lactic acid or glycolic acid and is ring-opening polymerized to give polylactide or polyglycolide having high molecular weight. Although the process affords high-molecular weight polymers, the two-step reaction entails great amounts of energy and is not economical. Direct polycondensation of lactic acid or glycolic acid is economically efficient but does not produce high-molecular weight polymers.
Patent Document 1 discloses a process in which polylactic acid of low molecular weight obtained by direct polycondensation is reacted with diisocyanate and thereby the molecular chains are extended. However, the process requires that the reaction with diisocyanate is performed at or above the melting point of the polylactic acid, for example at 210 to 215° C., and therefore the reaction is very difficult to control and has often resulted in evaporation of diisocyanate or undesired side reactions. Further, the process is only capable of extending the molecular chains to approximately twice the molecular weight of the low-molecular weight polylactic acid. It has been thus difficult to increase the molecular weight to more than twice that of low-molecular weight polymers.
Non-patent Document 1 describes a process for polymerizing low-molecular weight polylactic acid to more than twice the molecular weight thereof. According to this process, diol-terminated (i.e., at both ends) telechelic polylactic acid is reacted with diisocyanate, and the newly formed bonds are all thermally labile urethane bonds.
In addition to the above process, Non-patent Document 1 further discloses a process in which dicarboxylic acid-terminated (i.e., at both ends) telechelic polylactic acid having low molecular weight is reacted with a bisoxazoline compound. However, the bisoxazoline compounds are expensive and the process is not suited for industrial use.
Patent Documents 2 and 3 and Non-patent Document 1 describe other processes for producing polylactic acid resins having high molecular weight. However, these processes involve reactions at high temperatures and have similar problems as described above. Furthermore, the obtainable polylactic acid resins often have problems such as low crystallinity or coloration.    [Patent Document 1] JP-A-H05-148352    [Patent Document 2] JP-A-2002-155197    [Patent Document 3] JP-A-2004-285121    [Non-patent Document 1] Jukka Tuominen, CHAIN LINKED LACTIC ACID POLYMERS, POLYMERIZATION AND BIODEGRADATION STUDIES, Polymer Technology Publication Series Espoo 2003, Finland, Helsinki University of Technology, 2003, 2.28, No. 25
It is an object of the present invention to provide high-molecular weight and high-crystallinity polyester resins having an amide bond in the molecular chain, and production processes therefor and uses of the polyester resins. In more detail, it is an object of the invention to provide biodegradable polyester resins having an amide bond in the molecular chain, and production processes therefor and uses of the polyester resins.
The present inventors studied diligently to achieve the above object. They have then found that a polyester resin (C) having an amide bond in the molecular chain that is obtained through a specific reaction step possesses high molecular weight and high crystallinity. They have further found that the specific reaction step allows for inexpensive production of biodegradable polyester resins (C) having high molecular weight and excellent heat stability. The present invention has been completed based on the findings.
A polyester resin (C) according to the present invention is obtained by reacting an aliphatic polyester resin (A) and a polyisocyanate compound (B) in the presence of an amidation catalyst and comprises a structural unit represented by Formula (1) below:

wherein R is a C1-20 aliphatic hydrocarbon group, an alicyclic structure-containing hydrocarbon group or an aromatic ring-containing hydrocarbon group.
The aliphatic polyester resin (A) is preferably obtained from a hydroxycarboxylic acid and is more preferably polylactic acid. The polylactic acid is preferably obtained from lactic acid, and the content of L-isomer or D-isomer in the lactic acid is preferably not less than 90%.
The polyester resin (C) preferably has a crystallinity in the range of 10 to 70% and a weight average molecular weight in the range of 100,000 to 1,000,000.
The polyisocyanate compound (B) is preferably an aliphatic diisocyanate compound and is more preferably a compound selected from the group consisting of hexamethylene diisocyanate, isophorone diisocyanate, 1,3-(bis-isocyanatomethyl)cyclohexane, bis(isocyanatomethyl)bicyclo-[2,2,1]-heptane and bis(4-isocyanatocyclohexyl)methane.
The polyester resin (C) of the invention may comprise in a major proportion a structural unit represented by at least one selected from Formulae (2) to (4) below:
As used herein, the words “in a major proportion” mean that the resin contains the structural units represented by at least one selected from Formulae (2) to (4) below in a proportion of not less than 60 wt %, and more preferably not less than 90 wt %.

wherein R1 each independently represent a substituted or unsubstituted C1-20 aliphatic hydrocarbon group and n is an integer ranging from 20 to 1500;

wherein R1 each independently represent a substituted or unsubstituted C1-20 aliphatic hydrocarbon group, R2 represents a substituted or unsubstituted C1-20 aliphatic hydrocarbon group, a C2-20 unsaturated hydrocarbon group or an aromatic hydrocarbon group, and n is an integer ranging from 20 to 1500;

wherein R1 each independently represent a substituted or unsubstituted C1-20 aliphatic hydrocarbon group, R2 represents a substituted or unsubstituted C1-20 aliphatic hydrocarbon group, a C2-20 unsaturated hydrocarbon group or an aromatic hydrocarbon group, and n and m are each independently an integer ranging from 20 to 1500.
In a preferred embodiment, the weight average molecular weight of the aliphatic polyester resin (A) is in the range of 5,000 to 100,000 and that of the polyester resin (C) obtained is in the range of 100,000 to 1,000,000 and is 3 to 200 times larger than the weight average molecular weight of the aliphatic polyester resin (A).
A process for producing the polyester resin (C) according to the invention comprises a step of reacting an aliphatic polyester resin (A) and a polyisocyanate compound (B) in the presence of an amidation catalyst.
The polyisocyanate compound (B) is preferably a diisocyanate compound.
The aliphatic polyester resin (A) is preferably an aliphatic polyester resin in which a terminal hydroxyl group has been converted to a carboxyl group.
In a preferred embodiment, the polyisocyanate compound (B) is added in a molar amount that is 0.8 to 2.0 times the moles of the aliphatic polyester resin (A).
The aliphatic polyester resin (A) preferably has a weight average molecular weight in the range of 5,000 to 100,000.
The aliphatic polyester resin (A) preferably has a Sn content of not more than 300 ppm.
The amidation catalyst preferably contains at least one metal selected from the Group I, II and III metals of the periodic table and more preferably contains magnesium or calcium.
In the process for producing the polyester resin (C), the reaction is preferably carried out in a twin-screw extruder.
A film according to the invention comprises the polyester resin (C).
A shaped article according to the invention comprises the polyester resin (C).